1. Field of the Invention
This invention is in the field of fluorine chemistry and more particularly in the field of direct fluorination.
2. Description of the Prior Art
Saturated perfluoroplyethers are of current interest for new material applications due to their unusual properties. Lack of chemical reactivity and thermal stability are their outstanding features. These have been described as equally stable as perfluoroalkanes and unaffected over extended periods of time. See Henne, A. L., Richter, S. B., J. Amer, Chem. Soc., 74, 5420 (1952); Henne, A. S., Smook, M. S., J. Amer, Chem. Soc., 72, 4378 (1950); Banks, R. E., "Fluorocarbons and Their Derivatives:, MacDonald and Company, Ltd., London (1970) pp. 162.
The only reported reaction of saturated perfluoropolyethers is chain cleavage at the ether linkage by aluminum chloride, at elevated temperatures and autogenous pressure, to produce acyl chloride and trichloromethyl end-groups. See Tiers, G. V. D., J. Amer. Chem. Soc., 77, 4837, 6703, 6704 (1955). These remarkable stabilities along with their interesting surface properties, viscosities and the broad liquid ranges of the low molecular weight compounds make saturated perfluoropolyethers attractive for numerous applications as solvents, hydraulic fluids, heat-transfer agents, lubricants, greases, sealants, elastomers and plastics. See Paciorek, K. J. L., Kaufman, J., Nakahara, J. H., Ito, T. I., Kratzer, R. H., Rosser, R. W., Parker, J. A., J. Fluorine Chem., 10, 277 (1977); McGrew, F. C., Chem. Eng. News, 45, 18 (Aug. 7, 1967); Eleuterio, H. S., J. Macromolecular Sci.--Chem., A6, 1027 (1972).
Synthetic methods have been limited in the preparation of saturated perfluoropolyethers. The most successful synthesis has been an anionic polymerization of perfluoroepoxides, particularly hexafluoropropylene oxide and tetrafluoroethylene oxide. See Hill, J. T., J. Macromolecular Science--Chem., A8, 499 (1974); Eleuterio, H. S., J. Macromolecular Science--Chem., A6, 1027 (1972). This synthetic procedure is a three-step scheme for saturated perfluoropolyether production involving oxidation of perfluoroolefins to perfluoroepoxides, anionic polymerization to acyl fluoride terminated perfluoropolyethers and conversion of acyl fluoride end-groups to unreactive end-groups by decarboxylation reactions or chain coupling photolytic decarboxylative reactions.
Other general synthetic methods of perfluoroether and perfluoropolyether production are the addition reactions of perfluorohypofluorite (R.sub.f --O--F) with perfluoroolefins, and the perfluoroperoxide (R.sub.f --O--O--R.sub.f) addition reaction with perfluoroolefins and electrolytic fluorination in anhydrous HF of the corresponding hydrocarbon ethers. See, respectively, Porter, R. S., Cady, G. H., J Amer. Chem. Soc., 79, 5625 (1967); Hohorst, F. A., Shreeve, J. M., J. Amer. Chem. Soc., 89, 1809 (1967); Hohorst, F. A., Shreeve, J. M., Inorg. Chem., 7, 624 (1968); Toy, M. S., Stringham, R. S., J. Fluorine Chem., 5, 25, 481, (1975); Roberts, H. L., J. Chem. Soc., 4538 (1964); Toy, M. S., Stringham, R. S., J. Fluorine Chem., 7, 375 (1976); Burdon, J., Tatlow, J. C., Adv. Fluorine Chem., 1, 129 (1960); Simons, J. H., U.S. Pat. No. 2,500,388 (1950), Chem. Abs., 44, 5236b (1950); Simons, J. H. Brit. Pat. No. 659,251 (1951), Chem. Abs., 46, 2934b (1952); Kauck, E. A., Simons, J. H., U.S. Pat. No. 2,644,823 (1953), Chem. Abs., 48, 6469h (1954); Kauck, E. A., Simons, J. H., U.S. Pat. No. 2,594,272 (1952), Chem Abs., 46, 6015a (1952).
Direct fluorination techniques have not been previously employed in the synthesis of perfluorocarbon ethers. This may be due to the fact that direct fluorination reactions involve elemental fluorine and are characterized by quick evolution of large quantities of heat, ignition and flaming which promote product decomposition, often with explosive violence. The inability to control direct fluorination reactions to produce high yields of the desired fluorinated reactant without concomitant fragmentation of the desired product has prevented direct fluorination from becoming a widely accepted method of fluorination. Because of these problems, a very diversified art has been developed to circumvent or obviate the use of fluorine gas by using inorganic metallic fluorides, hydrogen fluoride, or electrolytic cells where no free fluorine is provided.
A relatively recently developed process of direct fluorination, known as the LaMar process, has been used to fluorinate hydrogen-containing organic, organometallic and inorganic materials including polymers. In the LaMar process, the hydrogen-containing material is placed in a reaction chamber and an inert atmosphere such as helium is introduced. Fluorine gas or an inorganic fluoride is introduced into the inert atmosphere in a very low initial concentration such as not to exceed about 6% at the end of 30 minutes. The temperature is maintained at a uniform low temperature so as to avoid uncontrolled fluorination. The LaMar direct fluorination process is further disclosed in the following references, the teachings of which are hereby incorporated by reference. R. J. Lagow and J. L. Margrave, "Direct Fluorination of Organic and Inorganic Substances", Proc. Natl. Acad. Sci., 67, 4, 8A (1970); R. J. Lagow and J. L. Margrave, "The Controlled Reaction of Hydrocarbon Polymers with Elemental Fluorine," Polymers Letters, 12, (April, 1974); A. J. Otsuka and R. J. Lagow, "The Direct Fluorination of Hydrocarbon Polymers", J. Fluorine Chemistry, (May, 1974); U.S. patent application Ser. Nos. 718,128 (1968), 133,804 (1971), 133,803 (1971), 133,865 (1971).
The LaMar process has also been extended to the fluorination of polyethers, such as polyethylene oxide, to produce perfluoroether oligomers having terminal carboxylic acid groups. In this technique, a polyether is placed in an enclosed reactor and maintained at a temperature below its decomposition point and subsequently directly fluorinated with a source of elemental fluorine until a perfluorinated polyether with terminal carboxylic acid groups is formed. This technique is described in U.S. Pat. No. 4,113,772, issued Sept. 12, 1978.